1. Field of the Invention
The invention relates to coatings for metal objects that inhibit corrosion and other detrimental, environmentally induced degradation of the metal object.
2. Description of the Prior Art
Metals such as copper, iron, silver, aluminum, tin, zinc, and their alloys are susceptible to corrosion if subjected to air, water or a solvent or the like. For prevention of such corrosion, attempts have been made to use, for instance, organic inhibitors such as benzotriazole or organic paints such as epoxy resin, acrylic resin and the like.
Use of an organic inhibitor such as benzotriazole on the metal surface has the shortcomings that it is partially dissolved by water, substantially solved in an acid or alkali, and evaporated at a high temperature as, for example, 80.degree. C. This will not prevent the metal surface from corroding over long periods of time.
On the other hand, if organic paint is used, it fails to make electric contact with metal due to it being an electrical insulator and having a high contact resistance. Moreover, the organic paint is apt to have pin holes so that localized corrosion develops through such pin holes. If the organic paint is subjected to a thermal shock, differential thermal expansion and contraction in relation to the metal causes deterioration of the bonding with the metal and loss of adhesion.
For example, a conventional ceramic dielectric resonator formed with a copper coating as inner and outer conductors is depicted in cross section in FIG. 1. The main body 1 of the ceramic dielectric resonator is formed of, for example, TiO.sub.2 ceramic dielectric in a cylindrical form. Inner and outer conductors (2 and 3 respectively) are formed in the inner and outer surfaces of the cylindrical main body 1. A connecting conductor 4 couples the inner conductor 2 to the outer conductor 3. A spring-like outer terminal 5 is fixed to the main body 1 by inserting and holding its spring portion in the opening of the main body. The resonator as shown constitutes a 1/4 wavelength coaxial resonator. The inner and outer conductors 2, 3 and the connecting conductor 4 are formed of a layer of copper produced by electroless plating.
The application of a benzotriazole film to the copper coating to form a corrosion protective coating was not entirely successful when the device was tested. The change in the Q value for such a device was more than 10% if the device was exposed for more than 1000 hours at a temperature of 80.degree. C., and a relative humidity of 85%.
The organic paint when used resulted in a bonding between the copper coating and the paint and between the other conductor 5 and the paint. When this combination was subjected to a hundred heat cycle test, each including a step for maintain the device at a temperature of -40.degree. C. for 30 min. and then for maintain the device at a temperature of 80.degree. C. for 30 min., the bond between the main body 1 and the copper coating deteriorated. The resonant frequency of a 800 MHz device changed in frequency more than 100 KHz. Exfoliation of the copper coating from the main body 1 was also observed. Application of the organic inhibitor or organic paint to the copper coating on the surface of the ceramic dielectric resonator has heretofore resulted in insufficient corrosion protection.
It is, therefore, an object of the invention to provide a metal corrosion protective coating for metals such as copper, iron, silver, aluminum, tin, zinc, a copper-zinc-tin alloy, a tin-zinc alloy, and the like.
Other objects and advantages of the invention will be apparent from the description of the preferred embodiments or may be learned form the practice of the invention.